The mucosal surfaces at the interface between the environment and the body have evolved a number of “innate defenses”, i.e., protective mechanisms. A principal form of such innate defense is to cleanse these surfaces with liquid. Typically, the quantity of the liquid layer on a mucosal surface reflects the balance between epithelial liquid secretion, often reflecting anion (Cl− and/or HCO3−) secretion coupled with water (and a cation counter-ion), and epithelial liquid absorption, often reflecting Na+ absorption, coupled with water and counter anion (Cl− and/or HCO3−). Many diseases of mucosal surfaces are caused by too little protective liquid on those mucosal surfaces created by an imbalance between secretion (too little) and absorption (relatively too much). The defective salt transport processes that characterize these mucosal dysfunctions reside in the epithelial layer of the mucosal surface.
One approach to replenish the protective liquid layer on mucosal surfaces is to “re-balance” the system by blocking Na+ channels and liquid absorption. The epithelial protein that mediates the rate-limiting step of Na+ and liquid absorption is the epithelial Na+ channel (“ENaC”). ENaC is positioned on the apical surface of the epithelium, i.e. the mucosal surface-environmental interface. Ideally, to inhibit ENaC mediated Na+ and liquid absorption, an ENaC blocker of the amiloride class will be delivered to the mucosal surface and maintained at this site to achieve maximum therapeutic benefit.
The use of ENaC blockers has been reported for a variety of diseases which are ameliorated by increased mucosal hydration. In particular, the use of ENaC blockers in the treatment of respiratory diseases such as cystic fibrosis (CF), and COPD, including chronic bronchitis (CB) and emphysema, which reflect the body's failure to clear mucus normally from the lungs and ultimately result in chronic airway infection has been reported. See, Evidence for airway surface dehydration as the initiating event in CF airway disease, R. C. Boucher, Journal of Internal Medicine, Vol. 261, Issue 1, January 2007, pages 5-16; and Cystic fibrosis: a disease of vulnerability to airway surface dehydration, R. C. Boucher, Trends in Molecular Medicine, Vol. 13, Issue 6, June 2007, pages 231-240.
Data indicate that the initiating problem in both CB and CF is the failure to clear mucus from airway surfaces. The failure to clear mucus reflects an imbalance in the quantities of mucus as airway surface liquid (ASL) on airway surfaces. This imbalance results in a relative reduction in ASL which leads to mucus concentration, reduction in the lubricant activity of the periciliary liquid (PCL), mucus adherence to the airway surface, and failure to clear mucus via ciliary activity to the mouth. The reduction in mucus clearance leads to chronic bacterial colonization of mucus adherent to airway surfaces. The chronic retention of bacteria, inability of local antimicrobial substances to kill mucus-entrapped bacteria on a chronic basis, and the consequent chronic inflammatory response to this type of surface infection, are manifest in CB and CF.
There is currently a large, unmet medical need for products that specifically treat the variety of diseases which are ameliorated by increased mucosal hydration, including CB, COPD and CF, among others. The current therapies for CB, COPD and CF focus on treating the symptoms and/or the late effects of these diseases. However, none of these therapies effectively treat the fundamental problem of the failure to clear mucus from the lung.
R. C. Boucher, in U.S. Pat. No. 6,264,975, describes the use of pyrazinoylguanidine sodium channel blockers for hydrating mucosal surfaces typified by the well-known diuretics amiloride, benzamil, and phenamil. However, these compounds are relatively impotent, considering the limited mass of drug that can be inhaled to the lung; (2) rapidly absorbed, and thereby exhibiting undesirably short half-life on the mucosal surface; and (3) are freely dissociable from ENaC. More potent drugs with longer half-lives on the mucosal surface are needed.
Too little protective surface liquid on other mucosal surfaces is a common pathophysiology of a number of diseases. For example, in xerostomia (dry mouth) the oral cavity is depleted of liquid due to a failure of the parotid sublingual and submandibular glands to secrete liquid despite continued Na+ (ENaC) transport mediated liquid absorption from the oral cavity. Keratoconjunctivitis sira (dry eye) is caused by failure of lacrimal glands to secrete liquid in the face of continued Na+ dependent liquid absorption on conjunctional surfaces. In rhinosinusitis, there is an imbalance between mucin secretion and relative ASL depletion. Failure to secrete Cl— (and liquid) in the proximal small intestine, combined with increased Na+ (and liquid) absorption in the terminal ileum leads to the distal intestinal obstruction syndrome (DIOS). In older patients excessive Na+ (and volume) absorption in the descending colon produces constipation and diverticulitis.
The published literature includes a number of patent applications and granted patents directed toward pyrazinoylguanidine analogs as sodium channel blockers. Examples of such publications include PCT Publication Nos. WO2003/070182, WO2003/070184, WO2004/073629, WO2005/025496, WO2005/016879, WO2005/018644, WO2006/022935, WO2006/023573, WO2006/023617, WO2007/018640, WO2007/146869, WO2008/031028, WO2008/031048, and U.S. Pat. Nos. 6,858,614, 6,858,615, 6,903,105, 6,995,160, 7,026,325, 7,030,117, 7,064,129, 7,186,833, 7,189,719, 7,192,958, 7,192,959, 7,192,960, 7,241,766, 7,247,636, 7,247,637, 7,317,013, 7,332,496, 7,345,044, 7,368,447, 7,368,450, 7,368,451, 7,375,107, 7,388,013, 7,399,766, 7,410,968, 7,745,442, 7,807,834, 7,820,678, 7,842,697, 7,868,010, 7,875,619, 7,956,059, 7,981,898, 8,008,494, 8,022,210, 8,058,278, 8,124,607, 8,143,256, 8,163,758, 8,198,286, and 8,211,895.
There remains a need for novel sodium channel blocking compounds with enhanced potency and effectiveness on mucosal tissues. There also remains the need for novel sodium channel blocking compounds that provide therapeutic effect, but minimize or eliminate the onset or progression of hyperkalemia in recipients.